Understanding Security Policy Files
The easiest way to learn to read security policy files is by looking at existing security policy files. In this section, you will get a closer look at the custom SharePoint security policy files WSS_Minimal and WSS_Medium. You can find them by looking at the policyFile attribute of the <trustLevel> element of a SharePoint Web.config file. By default, the SharePoint policy files are located in the CONFIG folder of the SharePoint hive: [drive letter]:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12.
The SecurityClasses and CodeGroup Sections
The first interesting bit of a security policy file is the <SecurityClasses> section, which contains a collection of <SecurityClass> elements. <SecurityClass> ...